The San Marcos salamander (Eurycea nana) is a small species of aquatic, lungless salamander native to the United States, endemic to Spring Lake and a small region of the headwaters of the San Marcos River near Aquarena Springs, in Hays County, Texas. It is one to two inches long, with a slender body and external gills, and is a reddish-brown in color. Description E. nana is an aquatic salamander. It does not leave the water to change into a terrestrial form, but rather matures in the water. As a neotenic form, E. nana retains its gills for its lifetime. The San Marcos salamander has a narrow head with a round snout, large eyes, and a dark iris. External gills are developed and pigmented. 2-6 palatopterygoid and 7-13 premaxillary teeth are present. The species is uniformly light brown in color, with a series of seven to nine irregular light spots present along its midline. The trunk is flattened above with a dorsal furrow that extends from head to tail. The venter, the bottom of a salamander, is white in color, and males have larger vents than females do. There are 16-17 costal grooves present, with 6-7 occurring between limbs. It has four toes on its forefeet and five on its hind feet. The size of the species was measured to be 56 mm total in length. Taxonomy The name nana is from the Greek nanos, meaning dwarf, as these adult salamanders are small in size. It is a member of the family Plethodontidae (lungless salamanders). Genetic variation of the species has not yet been observed. E. nana may be confused with E. neotenes, the only other species thus found to be in its genus; however, E. nana is smaller in size with a more slender form. Distribution San Marcos River, located in Hays County, Texas, is the only known location of the San Marcos salamander. The population estimate in 1973 was about 20,880 salamanders living in the uppermost reaches of Spring Lake. An estimate in 1993 suggested about 30,451 salamanders across all ranges of Spring Lake and up to 150 m downstream. The ratio of males to females was reported to be stable throughout the year. Habitat Clear, flowing spring water of the San Marcos River makes a well-delineated hydrologic system for the San Marcos Salamander. The springwater maintains a temperature of 21~22 °C; the salamanders appeared to be stressed at